The present invention is directed, in general, to apparatus for engraving coded data on a rotating disc, and more particularly to a rotor for supporting the code disc and to the drive mechanism and circuitry for rotating the disc.
The techniques for producing high resolution, precise patterns on code discs and the like are highly developed, and the procedures for producing such patterns on materials such as film, plastic, quartz, glass or other substrates are well known. For example, drafting techniques are available, wherein a master drawing of the code pattern is produced which is then photographically reduced to its final size. A further example is the use of metal backed papers and coated glass scribing materials, together with devices which guide the marking tools in precise X and Y coordinates, by which it is possible to produce patterns as large as four feet square with an accuracy of a few thousandths of an inch. Patterns produced by such methods, while somewhat limited in ultimate accuracy, are practically unlimited in design capability.
Another technique is to produce the coded patterns mechanically by means of precision "dividing engines" wherein optically polished diamonds are used to cut fine, parallel-sided lines through an opaque coating on polished glass master blanks. Such machines are capable of producing a divided circle that is accurate to one part in a million. Such machines usually limit the choice of pattern to relatively fine, radial, parallel, or circular lines or to FIGS. that can be applied to the lines by pentagraph methods. However, by mounting small optical projectors over the platens of such dividing engines, it is possible to project other pattern shapes onto the surface to be engraved, and by stepping the plate at precise and discrete intervals and turning the projector lamp on only when the engine is holding the plate stationary, additional patterns can be produced.
Another technique for producing encoded discs is described in U.S. Pat. No. 4,406,939 wherein a code pattern is inscribed on the surface of a coated disc by means of a laser beam transmitter. The disc is precisely positioned in relation to the laser beam transmitter by an angular step control device and by mounting the disc on the shaft before engraving the pattern, the pattern will be precisely centered on the disc. To obtain synchronization between the laser and the position of the disc, the disc drive motor and the laser beam transmitter are operated under the control of the same clock pulse generator. The motor may be a synchronous motor with its operating frequency derived from the clock pulse generator by a frequency divider. Such devices are limited in their accuracy, however, by the accuracy of the drive motor and the mechanical connections between that motor and the disc to be engraved. Prior devices also suffer a reduction in accuracy during the course of engraving a coded disc, because of the accumulation of errors which, even if minor to begin with, can gradually result in a loss of the required precision.